The present invention relates to a two cycle crankcase scavenged internal combustion engine, and in particular to a system for enhancing vaporization of the fuel-air charge and for more thoroughly scavenging residual hot gases from the combustion chamber.
In two cycle engines, residual exhaust gas remains in the combustion chamber due to incomplete scavenging. The residual exhaust gas is at a high temperature so that the subsequent charge of fuel-air mixture entering the combustion chamber is heated by the residual gas and produces radicals. The presence of radicals in the combustion chamber produces an active thermoatmosphere, which causes engine run-on, also known as active thermoatmosphere combustion, extraordinary combustion or self ignition. Self ignition can occur over a fairly broad speed and load range in low compression engines, for example, having a compression range of 6:1-8:1, without the combustible air-fuel mixture being ignited by a spark plug. Larger displacement engines are more susceptible than small displacement engines because of differences in combustion chamber compactness and the consequent inability to adequately scavenge the hot exhaust gases in the larger displacement engines, particularly when the engine is operated under part throttle speed and load conditions.
On the fringes of run-on, erratic operation of the engine, backfiring, misfiring and severe knock can result, all of which are very undesirable and may cause internal damage to the engine. Under light load or wide open throttle operating conditions, there may be some run-on but not enough to sustain engine operation without sparkplug ignition. This results in some cycles of run-on being interspersed with normal combustion cycles, thereby resulting in uneven engine operation.
A further problem with run-on is that a conventional ignition interrupt switch will not stop the engine when it is operating in a run-on condition, because under run-on conditions, there is no necessity for an ignition spark except during starting and warm up. Although an engine operating under run-on conditions can be stopped by altering the speed, load or fuel mixture or totally interrupting the flow of fuel to the combustion chamber, there is a certain amount of delay before the engine stops. Furthermore, this type of shut off system is more complex than the simple ignition shut off switches presently used.
It is relatively common for air cooled two cycle engines to experience exhaust port blockage and deposits in the combustion chamber after a certain number of hours of operation, and both of these conditions can cause run-on to occur. Ring seating will also make run-on more likely. Thus, run-on is more prevalent the longer the engine has been operating.
A rich fuel-air mixture is effective in preventing run-on, so if the engine is equipped with a fixed main and idle carburetor, it is possible to factory set the mixture rich enough to eliminate the possibility of run-on under normal operating conditions. However, even if such a carburetor is used, leaning of the mixture may later occur due to a number of causes. For example, the air vent to the fuel tank or reservoir might become partially obstructed, which would cause a partial vacuum to occur in the tank thereby reducing the flow of fuel to the carburetor and resulting in a leaner mixture. Gum or debris may partially block the fuel filter or screen or gum or debris may partially block the fuel metering passages, thereby also resulting in a leaner mixture. Another possible cause of lean mixture is air leakage into the crankcase or induction system caused by damaged or worn crankcase seals, damaged castings, hardware inadequately tightened when servicing or hardware loosened due to vibration. Any one of these conditions can cause the mixture to become sufficiently lean that run-on conditions occur.
An adjustable main and idle carburetor may result in the engine being adjusted by the user to the point where run-on conditions occur. Adjusting the carburetor so that the engine runs leaner results in smoother and quieter operation of the engine. However, a leaner fuel-air mixture also results in less lubricant entering the engine with the fuel thereby reducing lubrication of the piston and causing piston ring sticking, scuffing or scoring. Furthermore, if the mixture is leaned out too far, run-on conditions will occur and the conventional ignition switch will not be effective to stop the engine.
A further problem with residual exhaust gas and resulting pre-ignition is that the cylinder does not cool to the same extent as under normal combustion conditions. This results in higher piston temperatures thereby causing scuffing, scoring and piston ring sticking.
Another problem experienced with conventional crankcase scavenged two cycle engines is that of incomplete vaporization of the fuel. In such engines, the areas and volumes change drastically from the carburetor to the combustion chamber, and additionally, intake valving such as reed, rotary or poppet valves and piston ports operate as flow barriers which tend to condense the fuel vapor into droplets. Moreover, the rotation of the crankshaft within the crankcase creates a windage which tends to direct droplets and also the vapor onto the crankcase walls. Unless this condensed liquid fuel is vaporized in the crankcase, it enter the combustion chamber in a liquid state, is difficult to ignite thereby resulting in slow burning or missing.
When a two cycle crankcase scavenged engine is first started and the engine is cold, the condensed liquid fuel accumulates in a low spot in the crankcase and results in a condition commonly known as "puddling". Because a significant portion of the fuel is not drawn into the combustion chamber, the mixture becomes excessively lean thereby resulting in poor ignition. Although this lean starting condition can be partially offset by choking the carburetor to create a rich mixture for starting, it is impossible to achieve an optimum mixture for ignition until the combustion chamber and crankcase have warmed up adequately to cause proper fuel vaporization. During this warm up period, the engine is noisy, runs rough and smokes excessively due to the poor ignition of the mixture. This problem is particularly prevalent in engines used in cold weather applications such as snowthrowers, ice augers and snowmobiles. If the carburetor is calibrated to supply a richer mixture for the cool conditions, it will be excessively rich when the engine is at normal operating temperatures.
It has been found that the recirculation of warm, inert gas, such as uncombustible exhaust gas, into the crankcase enhances fuel vaporization, thereby making the fuel-air mixture more combustible. It is known to inject a charge of exhaust gas into the crankcase, but in that case, the exhaust gas was taken directly from the exhaust manifold, and it is therefore hot and contains a substantial amount of uncombusted fuel. The recirculation of very hot exhaust gas causes excessive crankcase temperatures which can result in breakdown of the lubricant thereby causing bearing failures. Also, the recirculation of exhaust gas that has a substantial amount of uncombusted fuel will cause the deposit of particulate matter on the recirculation port thereby blocking the port.